This invention relates to microelectromechanical systems and/or nanoelectromechanical systems (collectively hereinafter “microelectromechanical systems”) and techniques for fabricating microelectromechanical systems; and, more particularly, in one aspect, for adjusting, tune, setting, defining and/or selecting the output frequency of microelectromechanical resonators.
Many conventional micromechanical structures are based on the reaction (for example, oscillation, deflection or torsion) of a beam structure to an applied force. Such beam structures are often fabricated from monocrystalline or polycrystalline semiconductors. These materials have excellent mechanical strength and a high intrinsic quality factor.
However, manufacturing and material tolerances regarding such micromechanical structures often significantly impact the operation and/or function of the structure. For example, microelectromechanical resonators typically rely upon the bending beam and lateral oscillating beam structures. In this regard, manufacturing and material tolerances tend to lead to large variations of the resonance frequency of such resonators.
Notably, the beam structures of conventional resonators are often rectangular in shape and/or cross section. The mechanical stiffness (kM) of a beam, as calculated with respect to the oscillation direction parallel to the width of the beam (w), is proportional to its Young's modulus (E) and certain measures of its geometry, including for a beam with a rectangular cross section, length (L) and height (h). That is:
                              k          M                ≈                              E            ·            h            ·                          w              3                                            L            3                                              EQUATION        ⁢                                  ⁢        1            
Setting aside electrostatic forces, the resonance frequency (f) of a beam may thus be defined under these assumptions by the equation:
                    f        ≈                              1                          2              ·              π                                ·                                                    k                M                                            m                eff                                                                        EQUATION        ⁢                                  ⁢        2            
where meff is the effective mass for the resonating mode shape of the beam.
Thus, as is apparent from EQUATIONS 1 and 2, the dimensions and effective mass of the (moving) beam or electrode of a microelectromechanical resonator, and the capability of precisely and repeatedly controlling such dimensions and mass, are critical to accurately predicting (and/or controlling) the resonance frequency of the resonators.
Although conventional processes for manufacturing microelectromechanical resonator are relatively precise, such processes have inherent engineering or processing tolerances. Similarly, the tolerance/range of the material's characteristics and/or properties produce differences in Young's modulus, which are difficult to exactly predict, compensate, address and/or predetermine before fabrication. Such manufacturing and material tolerances may lead to relative large frequency variations even between identically designed resonators. Indeed, such frequency variations may be measurable (and likely unacceptable) even between microelectromechanical resonators that undergo identical processing/fabrication, for example, resonators fabricated on the same wafer.
There have been many attempts to address the issue of variations in the resonant frequency of a microelectromechanical resonator due to manufacturing and material tolerances. For example, one such technique employs a laser to vaporize material away from an open, unpackaged oscillator. In this technique, prior to packaging, the output frequency of the oscillator is measured. If adjustment is required to provide a particular output frequency, material disposed on the oscillator is removed in order to increase the frequency of the output. (See, for example, Noell et al., “MEMS for Watches”, IEEE, 2004 0-7803-8265-X/04, pages 1–4). Once a suitable frequency is attained, the oscillator is packaged.
Another technique employs a laser to “move” a metal material from a nearby “sacrificial” metal layer onto the mechanical structure (comprised of a silicon material) of the microelectromechanical resonator. See, for example, Chiao and Lin, “Post-Packaging Tuning of Microresonators by Pulsed Laser Deposition”, IEEE 2003, 0-7803-7731-1/03, pages 1820–1823. Here, the microelectromechanical resonator is hermitically sealed, via wafer bonding, with a glass wafer cap. The output of a laser is applied through the glass cap and incident on a metal layer disposed in the sealed chamber. The metal material is “blasted” off the cap and onto the oscillator mass. In this way, additional mass is deposited on, for example, the moving beam(s) or electrode(s), which decrease the resonant frequency of the microelectromechanical resonator.
Such a configuration, and, in particular, the glass wafer packaging and sacrificial metal layer, may, among other things, adversely impact the vacuum of the microelectromechanical resonator, particularly in view of the evaporation of the metal material within the resonator chamber. Such evaporation may produce unwanted contaminants within the chamber which, in turn, may adversely impact the operation of the resonator.
Thus, there is a need for a technique for tuning, setting, defining and/or selecting the output frequency of microelectromechanical resonator that overcomes one, some or all of the shortcomings of conventional techniques. There is a need for a technique that compensates for, and/or addresses, minimizes and/or eliminates the adverse affects of manufacturing and material tolerances of microelectromechanical resonator without contaminating the chamber of the resonator. Notably, it may be advantageous if such a technique does not rely on the incorporation of materials and/or techniques that are incompatible, or difficult to integrate with CMOS circuits.